Crack Paths 2012

The specimens were tested in a servo-hydraulic test rig under axial load (loading

direction parallel to the x axis) under two load ratios with and without overloads. In the

case of overloads, a periodic tensile overload with overload ratio O L R= 60% was

introduced. For both load ratios RS = -1 (fully reversed loading) and RS = 0 (pulsating

loading) the overload ratio was defined according to Eq. 1 [6].

100⋅¸¸¹·¨¨©§−=aaOLSSS O L R

(1)

In Eq. 1, Sa denotes the maximumstress in constant amplitude loading cycles and SOL

the overload peak. The overload was introduced in the middle of each block of 1000

cycles, which means the first overload was applied after 500 cycles and afterwards with

a pace of 1000 cycles. Representative spectra for the load ratio RS = -1 und RS = 0 are

shown in Figures 4a and 4b respectively.

Fig. 4: Loading spectrum with overloads by load ratio RS = -1 (left) and RS = 0 (right).

The tests were carried out till the specimen failure, with run-out limit fixed at 5·106

cycles. The fracture surfaces were analyzed using a scanning electron microscope with

an accelerating voltage of 20 kV and a working distance between 15-16 mm.The crack

surface of specimens of as-received and flange material states of both tested load ratios

RS = -1 and RS = 0 (with and without overload) are investigated.

R E S U L T S

Fatigue Tests

The stress-life curves derived in the fatigue tests are depicted in Figures 5 - 8 and

summarised in Table 2. After the knee-point a constant slope of the Wöhler-curve k’ =

44.9, correspondent to a decrease of 5 %per decade was assumed [7].

704